Conventionally, at the scene of manufacture, a robot referred to as a power assist device is used so as to reduce labor and improve workability. As a control method of the power assist device, mechanical impedance control based on a position control (called an admittance control) is widely adopted.
In the admittance control, normally, position control with a high gain is performed and its target position is determined based on a force sensor. Then, force applied on the power assist device not through the force sensor is not considered at the control action, and a manipulator is hardly actuated. Namely, when the power assist device is brought into contact with an environment (the external) at the position except the detection part of the force sensor, the power assist device is continued to be actuated without consideration of the contact.
Hereafter, situations where humans and robots share a space will increase more and more so that the admittance control will consider the connection of a robot with a person or an environment
The Non-patent Literature 1 shown below discloses a method of the admittance control with consideration of connection with an environment by using estimated value of external force acting on the power assist device not via any force sensor.
In the control method with consideration of the connection with the environment disclosed in the Non-patent Literature 1, compliant action following unknown external force can be realized. However, in this control method, there is known that, when an operator brings the robot into contact with the environment and then presses the robot to the environment further, the robot vibrates along the pressing direction and the anti-pressing direction (so-called oscillation phenomenon occurs). Essentially, the oscillation phenomenon should not occur in the state that the operation force is perfectly balanced with the external force. However, even slight difference between the two causes the oscillation phenomenon. The oscillation phenomenon becomes a problem in the case that the connection of the robot with the environment is premised such as pressing work with a power assist device.
For solving the problem, hitherto, such a method that an operator adjusts pressing condition of the power assist device is only obtained. With regard to parameters concerning the action of the power assist device, viscosity resistance is set to be increased so as to suppress the oscillation phenomenon. However, in this case, another problem occurs that the operation force required for the operation of the power assist device is increased so as to worsen the operability, whereby it is undesirable in practical use.
Namely, with regard to the conventional art, there is no method effective to suppress the oscillation phenomenon caused in the pressing work with the power assist device.
[Non-patent Literature 1]: Masahito Iso, Hirokazu Seki, and Yoichi Hori, Sensorless power assist method using impedance control considering load characteristics, Technical Meeting on Industrial Instrumentation and Control, IEE Japan, Vol. IIC-02-40 (2002)